Metabolic syndrome II: obesity

Sweetness and light

Low-calorie sweeteners may make people fat

ARTIFICIAL sweeteners have long been touted as being good for the calorie-conscious. Unfortunately, a study just published in Behavioral Neuroscience by Susan Swithers and Terry Davidson of Purdue University in Indiana suggests that such compounds may actually end up making people fatter than they otherwise would be.

Dr Swithers and Dr Davidson came to this conclusion after a series of experiments on rats. In addition to standard laboratory food, they fed some animals yogurt flavoured with saccharin, while others had yogurt flavoured to the same degree of sweetness with normal sugar.

The researchers then carried out two experiments on their animals. One merely tracked the rats' weight over five weeks. This found that rats eating sweetener gained more weight than those eating sugar. The other experiment was more subtle. After two weeks on yogurt, the rats were given an unexpected treat—a chocolate pudding loaded with calories. Both groups gobbled this up. However, those animals that had been eating sugared yogurt reduced the amount of yogurt they ate for their next meal in proportion to the number of chocolate-flavoured calories they had consumed. Those on the sweetener made no such adjustment.

Dr Swithers and Dr Davidson also measured the body temperatures of their charges before and during the chocolate meal. In normal animals the brain raises the body's temperature before and during eating. This is to prepare for the energy-intensive job of digestion. As expected, those rats on the sugar diet showed a normal temperature rise. Those on the sweetener, however, showed a diminished increase in temperature, suggesting that their physiology was in some way affected.

The cause, Dr Swithers and Dr Davidson think, is a disruption in the connection that the brain makes between sweetness and calories. Past research suggests that the brain thinks that sweetness is a sign of highly calorific food. Dr Swithers and Dr Davidson argue that artificial sweeteners confuse things. After repeated exposure to sweeteners, the brain forgets the connection and thus fails to stop the animal eating at an appropriate point.

Rats are not people, of course—though what holds physiologically for one often holds for the other. Nor do these experiments suggest that sweeteners literally make people fat. Unlike rats, people are rational. If sweeteners are used as part of a low-fat diet in which all calories are accounted for, there should be no problem. If...

It therefore looks possible that low-calorie artificial sweeteners are a contributory factor to the rising number of people who are obese. What an irony.